INTEREST has recently been expressed in the possibility that the interstellar dust contains a silica component (ref. 1, and R. C. Gilman, private communication). The purpose of this communication is to point out that silicon dioxide has strong absorptions in the 8–14µ atmospheric “window” which should be observable with ground-based infrared telescopes. We have computed normalized extinction spectra (extinction cross-section divided by geometrical cross-section) for crystalline quartz and vitreous silica.